2025ApJ...991..102P

Radio relics, arc-like polarized sources with highly aligned magnetic fields, are typically found on the outskirts of merging galaxy clusters. The magnetoionic media responsible for the significant coherence observed in radio relics remain poorly understood. Low-frequency measurements of radio relics are essential for constraining depolarization models, which provide crucial insights into the magnetic field distribution. However, these measurements are challenging due to the emission properties and interferometer systematics. We have detected polarization signals from the northwest radio relic in A746 at 650 MHz with the upgraded Giant Meterwave Radio Telescope, marking the first-ever detection of polarization from radio relics below 1 GHz. At this frequency, the average rotation-measure-corrected magnetic fields align well with shock radio emission, typical of radio relics. The fractional polarization at 650 MHz is ∼18% ± 4%. Our results indicate that a single internal depolarization model cannot explain the observed depolarization spectra, suggesting a nonuniform magnetic field distribution or complex contribution of different polarized regions in the radio relic. Our detection of polarization signals at 650 MHz reveals critical insights into radio relic magnetic field structures, offering a low-frequency approach to understanding intracluster medium magnetic fields in merging galaxy clusters.